prostaglandin-d2 and dazoxiben

Selective pharmacological blockade of thromboxane-synthase in human platelets by dazoxiben resulted in the reorientation of cyclic-endoperoxides towards PGE2, PGD2 and PGF2 alpha. At concentrations which can be reached when thromboxane-synthase is inhibited, PGE2 (100-500 nM) exerted a marked, concentration-dependent pro-aggregatory effect. This required the formation of endogenous or the addition of exogenous endoperoxides and was prevented by PGD2 or 13-aza-prostanoic acid, a selective antagonist of PGH2/TxA2 receptors. The anti-aggregating effect of PGD2 was evident at concentrations lower than those obtained in dazoxiben-treated platelets. It is proposed that in the absence of TxA2 generation, a combination of endoperoxides and PGE2 may result in normal aggregation. The latter may be inhibited by PGD2. No interference of PGF2 alpha on platelet function could be shown.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Adenosine Diphosphate; Arachidonic Acid; Arachidonic Acids; Aspirin; Blood Platelets; Dinoprost; Dinoprostone; Drug Synergism; Humans; Imidazoles; Oxidoreductases; Platelet Aggregation; Prostaglandin D2; Prostaglandin Endoperoxides; Prostaglandin Endoperoxides, Synthetic; Prostaglandins; Prostaglandins D; Prostaglandins E; Prostaglandins F; Prostanoic Acids; Thromboxane B2; Thromboxane-A Synthase

Prostanoid synthesis and release during collagen-induced aggregation of human platelet rich plasma (PRP) was studied using a novel gas chromatography/mass spectrometry assay technique. Aggregation was associated with the production of mainly thromboxane A2 (TXA2), measured as TXB2, and smaller amounts of the prostaglandins (PGs) D2, E2 and F2 alpha. UK 37,248 inhibited TXB2 formation by greater than 95% and increased the production of PGD2, PGE2 and PGF2 alpha twenty-fold. The relative amounts of these three prostanoids were not changed by UK 37,248. Even though high concentrations of PGD2 were formed, aggregation was not inhibited. In contrast, flurbiprofen inhibited aggregation, demonstrating that platelet aggregation produced by this concentration of collagen is cyclooxygenase dependent. These results support the proposal that the prostaglandin endoperoxides can induce aggregation alone, irrespective of the amount of PGD2 that is produced.

Topics: Blood Platelets; Collagen; Cyclooxygenase Inhibitors; Dinoprostone; Gas Chromatography-Mass Spectrometry; Humans; Imidazoles; In Vitro Techniques; Platelet Aggregation; Prostaglandin D2; Prostaglandin Endoperoxides; Prostaglandins D; Prostaglandins E; Thromboxane-A Synthase

This study shows that dazoxiben, a selective inhibitor of thromboxane A2-synthetase in human platelets, inhibited arachidonic acid-induced platelet aggregation in platelet-rich plasma samples from four out of 16 healthy volunteers. In these four "responder" samples, the anti-aggregating effect of dazoxiben was prevented by the compound SQ 22536, a 9-substituted adenine analogue, endowed with an inhibitory activity on adenylate-cyclase. The compound SQ 22536 also counteracted the antiaggregating effect of prostaglandin D2, a known activator of platelet adenylate-cyclase. When platelet thromboxane A2-synthetase was blocked by dazoxiben, a marked increase of prostaglandin D2 was concomitantly observed both in "responder" and "non responder" samples. The compound SQ 22536 blunted the increase in platelet cAMP caused by either dazoxiben and sodium arachidonate or prostaglandin D2. It is suggested that the antiaggregating effect of dazoxiben is mediated by newly synthesized prostaglandin D2. The latter acts by stimulating adenylate-cyclase and increasing cAMP levels. The compound SQ 22536 prevents both phenomena. In "non responder" samples some factors--still to be defined--might counteract similarly to the compound SQ 22536 the antiaggregating activity of PGD2.

Topics: Adenine; Adenylyl Cyclase Inhibitors; Blood Platelets; Cyclic AMP; Humans; Imidazoles; Oxidoreductases; Platelet Aggregation; Prostaglandin D2; Prostaglandins D; Thromboxane A2; Thromboxane-A Synthase

Dazoxiben , an imidazole-derived selective inhibitor of thromboxane A2 (TxA2) synthetase, prevented TxB2 synthesis in vitro in platelet-rich plasma from 16 normal subjects. Inhibition of TxB2 synthesis was accompanied by increased generation of PGE2, PGF2 alpha, and PGD2, as shown by radioimmunoassay, thin-layer radiochromatography, and high-resolution gas chromatography-mass spectrometry. Even at dazoxiben concentrations (40-80 microM) above those inhibiting TxB2 synthesis, platelet aggregation induced by threshold concentrations of arachidonic acid was inhibited in only 4 of 16 subjects, referred to as responders. The remaining 12 individuals were defined as nonresponders. The aggregating effect of arachidonic acid and of the prostaglandin-endoperoxide analog U-46619 was potentiated by PGE2 and prevented by PGD2 at concentrations within the range of those detected in dazoxiben -treated platelet-rich plasma. The antiaggregating effect of dazoxiben was counteracted by PGE2 (in responders) and was potentiated by PGD2 (in nonresponders). Platelets from responders and nonresponders did not differ in the amount of immunoreactive PGE2 material or in their sensitivity to U-46619 or PGD2. It is concluded that inhibition of thromboxane synthetase does not per se prevent platelet aggregation. The functional result of thromboxane suppression appears to be modulated by an interplay of the prostaglandin-endoperoxides, PGE2 and PGD2, which are formed in excess.

Topics: Adult; Arachidonic Acid; Arachidonic Acids; Chromatography, Gas; Chromatography, Thin Layer; Dinoprost; Dinoprostone; Female; Humans; Imidazoles; Male; Mass Spectrometry; Oxidoreductases; Platelet Aggregation; Prostaglandin D2; Prostaglandins D; Prostaglandins E; Prostaglandins F; Radioimmunoassay; Thromboxane A2; Thromboxane-A Synthase; Thromboxanes

Dazoxiben, a thromboxane synthase inhibitor, inhibits arachidonic acid induced aggregation in platelet-rich plasma from some donors only ("responders"). We have studied the effect of dazoxiben in vitro on platelet aggregation and prostaglandin (PG) metabolism and the influence of the incubation period and of exogenously added serum albumin (SA). SA, which increases the production of anti-aggregatory PGD2 from cyclic endoperoxides, induced "non-responder" human platelets to respond. With rabbit platelets, however, that are insensitive to PGD2, exogenous SA failed to potentiate dazoxiben-induced inhibition. The ratio between PGD2 and TXB2 + PGE2 formed was crucial in determining the response of human platelets to dazoxiben: whenever this ratio was high, platelet aggregation was inhibited. SQ 22536, an adenylate cyclase inhibitor, and NO164, a PGD2 antagonist, reversed the inhibition by dazoxiben in human platelet-rich plasma, stressing the importance of a PGD2 mediated rise of cyclic AMP for the effectiveness of a thromboxane synthase inhibitor.

Topics: Adenine; Adult; Arachidonic Acid; Arachidonic Acids; Blood Platelets; Female; Humans; Imidazoles; Male; Middle Aged; Oxidoreductases; Platelet Aggregation; Prostaglandin D2; Prostaglandins D; Serum Albumin; Thromboxane-A Synthase